In a network, network layer resources, such as bandwidth and other resources associated with the Network Layer (layer 3 or L3) of the Open Systems Interconnection Reference Model (OSI-RM) are limited. Demand for resources often outpaces supply. Network layer control protocols address this issue of supply and demand by reserving network layer resources necessary to provide network layer services, such as constant data rate and low-delay data transport, for real-time data transmission or communication, such as voice over Internet Protocol (IP) (VoIP) or video on demand (VoD). But again because network layer resources are limited, resources may not be available for each and every reservation requested. This is a case of offered load exceeding network link (or interface) capacity. As such, reservation protocols, such as Resource ReSerVation Protocol (RSVP) (defined in R. Braden, et al., “Resource ReSerVation Protocol (RSVP),” Request For Comments (RFC) 2205), often support preemption and prioritization of reservations.
Preemption of reservations generally relates to preempting an existing lower priority (preference) reservation in order to reallocate its resources to a new higher priority reservation. Prioritization of reservations generally relates to a priority given to a reservation with respect to allocating resources to the reservation over other reservations. Implementing reservation preemption and/or prioritization across an entire network, such as the Internet, however, is a difficult proposition at best.
The Internet is a loose confederation of networks controlled by many individual and separate entities and organizations—not just one. It is unlikely all will ever agree as to which reservation preempts or is preferred over another reservation. To do so, would permit one organization to dictate how scarce network layer resources are reserved by another organization. These policy decisions on what or which traffic deserves which packet treatment within a particular domain (or network) are matters for that local domain to make. Sometimes, two adjacent domains can agree on similar treatments for a type of traffic, but this is rare, and not expected to change soon.